Bennett Miller's Capote ends with a caption telling us that, after the publication of In Cold Blood (his 1966 work of crime reportage), Truman Capote never completed another book. The film recounts the making of a writer: although he was already famous, the best-selling In Cold Blood sealed Capote's reputation. But Capote is also about the making of a non-writer: it depicts a series of moral failures and spirit-sapping exertions that apparently made the book debilitating for its author (though it was more likely success that really did him in). Capote feels almost like a prequel to what could be a much more interesting and delicate story, about the life of a writer when he's not delivering.

For whatever reason, In Cold Blood became Capote's albatross, and you only worry that Capote might prove an albatross for its Oscar-nominated, Bafta-winning lead, Philip Seymour Hoffman. While Hoffman will no doubt give us dazzling performances for years to come, Capote is his breakthrough as a household name, and he's so remarkable in it that people will henceforth and forever impersonate Hoffman as Capote, just as they do Brando as Don Corleone. But Capote himself registers in the film as a man perpetually impersonating himself - which is certainly one definition of a celebrity.

We first meet the attention-hungry writer entertaining a crowd with a catty anecdote about James Baldwin, in which Capote of course gives himself the killer punchline. Whatever he may write about, the Capote we meet is virtually incapable of talking about anything other than himself, even reporting on a murder case. The film concentrates on Capote's visit to Kansas, accompanied by his friend Nelle Harper Lee (soon to be famous as the author of To Kill a Mockingbird), to cover the trial of two men for murdering a family. Once there, Capote can't resist flaunting his star status and dispensing gossip about what Humphrey (Bogart) said to John (Huston).

Hoffman's exotic, piss-elegant Capote is an exquisitely thorough physical performance. The actor embodies his subject right down to the hesitant flare of his nostrils; the way he cradles a glass or a cigarette with self-conscious thinkerly poise; walking with a scarf draped round his neck, college-boy style, he glides on invisible castors. Somehow, this bulkiest of actors - with the aid of Adam Kimmel's photography - shrinks unobtrusively into an ethereal literary pixie. As for the airy Southern tones, it's hard at first to get used to their archness. But the voice proves an eloquent vehicle for self-delight: at a public reading, Capote announces himself with a little amused chuckle, as if the very mention of his name is a delicious joke shared with the happy few.

The performance doesn't immediately gel with a context that's realist and understated in tone. What Hoffman gives us is partly a stylised impersonation of a public figure with an outlandish trademark manner. But if Capote's persona comes across as bigger and fancier than the world around him, that's the point: he seems like an unreal being who has been pasted onto reality, rather than inhabiting it.

Scripted by Dan Futterman, the film sensitively examines Capote's relationship with Perry Smith (an affecting Clifton Collins, Jr), the killer that he came to know closely. We see Capote solicitously dole out aspirin to Smith, then feed him baby food when he's weak. It's all the more shocking, then, that Capote later withdraws from Smith: by the time the two men's execution approaches, he virtually retreats into a babyish catatonia at the prospect of facing them again.

More generally, then, the story is about the responsibilities of writing, with Capote's betrayal of his subjects seen as the unacceptable price to be paid for his literary achievement: as time draws on, all he wants is to finish his book, which can only be brought to a satisfying ending if the killers are too. Few literary biopics have been so disillusioned about their subjects, and we cringe to see New Yorker editor William Shawn (Bob Balaban) practically frog-march Capote back to Kansas to face the two convicted men on the eve of execution. The film doesn't make a song-and-dance about Capote being a great writer; it's more concerned with the peculiar meaning that writing has in his life, on the one hand something he laughs off as almost a party trick, and on the other, an ordeal that he must endure in order to have a self. Capote is both enthralled and agonised by his expectations of himself: "Sometimes," he marvels, "when I think how good my book could be, I can hardly breathe."

The film is unforgiving about his narcissism: its most pointed moment shows a maudlin Capote spilling his woes to Nelle (Catherine Keener) at the premiere of the film based on To Kill a Mockingbird. She listens patiently, then coolly asks him how he liked the movie. It's a slim role for Keener, but she plays it with tender seriousness, and her timing as straight woman to Capote's stand-up farceur is perfect, establishing a complicity that feels plausible as a life-long relationship.

Keener apart, the great problem with Capote is that Hoffman's performance is so much richer than everything around it, which ultimately seems like a studious, respectful showcase for his flamboyance. Prepare yourself for a familiar solemnity of execution - the blue-grey atmospheric photography, the skeletal score of bleak piano and strings - that tells you that Oscar season is upon us.

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